In recent years, a substantial effort has been directed toward the application of solid state technology to transducer design and fabrication. As a consequence, semiconductor strain gauges have been developed which offer considerable advantages over conventional wire and foil gauges. Specifically, the semiconductor gauges may be embodied in units of relatively small size that are rugged and highly sensitive.
Various specific forms of semiconductor strain gauges have been proposed including heterojunction diodes utilizing two distinct semiconductor members of different conductivity type. See, for example R. Moore and C. J. Busanovich, IEEE Proc., April 1969, pages 735-736. Although such transducers or sensors have been quite successful, a major drawback of such devices is their limited strain sensitivity, poor temperature stability, complexity and high cost of construction. In the usual heterojunction diode, signal current increases when the diode is stretched. When attached to a substrate such as glass, the differential in expansion upon heating results in a threshold current which, together with increased thermal current, gives rise to a poor temperature stability characteristic. Also, the heterojunction devices are diodes and so must operate with a current of only one polarity.
Another type of strain sensor in current use is the high resistivity piezoelectric transducer wherein one senses the voltage induced by strain. The piezoelectric materials used in such devices include such naturally high resistance material as barium titanate and also materials such as cadmium sulfide which has been doped to a very high resistivity. The resistance is high enough so that internal charge movement under the influence of the strain induced field does not cancel out the charge induced by the strain at the electrical contacts (otherwise no voltage would be seen by the external sensing circuit). Since charge motion in the external circuit will also cancel induced charges, one must use a high input impedence detector and only time varying strains can be detected. The present invention overcomes the foregoing drawbacks and relies on a unique phenomenon to provide strain sensitive resistance operation using the piezoelectric material. Specifically, I have discovered that piezoelectric semiconductor materials can be prepared so as to have strain sensitive resistance along at least one crystal axis. For example cadmium sulfide can be processed so as to have a highly strain sensitive resistance along the C axis. By applying a voltage across the semiconductor material parallel to the direction of strain sensitive resistance, one can measure a signal current which is proportional to strain. Thus, in contrast to devices which function piezoelectrically, static strain forces can be measured. The transducers of this invention also have high sensitivity, as much as an order of magnitude greater than heterojunction diodes.
The transducers of this invention also provide a signal current which is decreased when stretched and increased when compressed. Accordingly, in contrast to heterojunction diodes thermally induced strain (resulting from expansion coefficient differentials) actually compensates for thermal current increases resulting in enhanced temperature stability. Furthermore, the transducers can be operated with an applied voltage of either polarity and with small A.C. voltages.